The present invention relates to a flat azimuth meter or bearing sensor having a plane coil laminated with thin film magneto resistive elements (hereinafter referred to as xe2x80x9cmagneto resistive elementsxe2x80x9d) and to a small and light azimuth meter suitable for mobile devices.
When a current is applied to a magneto resistive element in a direction of an easy axis of magnetization, and at the same time, a magnetic field is applied in a direction perpendicular thereto, an electric resistance in the current direction has a magneto-resistance effect, that is, it is reduced depending on a magnetic field strength. A relationship between the electric resistance (hereinafter referred to as xe2x80x9cresistancexe2x80x9d) and the applied magnetic field strength can substantially be shown as in FIG. 20.
Assuming Hk denotes a saturation magnetic field, when a biasing magnetic field on the order of xc2xdxc2x7Hk is applied to a magneto resistive element, there is a substantially linear relationship between an external magnetic field H and the resistance R. An external magnetic field can be measured by using the linear relationship between the external magnetic field H and the resistance R when a certain biasing magnetic field is applied. Then, when each of two components orthogonal to each other of the earth magnetism is detected by two groups of magneto resistive elements that an appropriate bias is applied to, bearings can be measured at a measuring point.
There is used an azimuth meter or a bearing sensor comprising an MR bridge constituted by four magneto resistive elements 91, 92, 93, and 94 that are orthogonal to each other as shown in FIG. 21, and two bias coils 101 and 102 that are wound around a holder mounted outside of the magneto resistive elements so that two orthogonal biasing magnetic fields can be applied both at an angle of 45 degrees with respect to the current directions of the magneto resistive elements. FIG. 22 is a schematic cross-sectional view thereof, and FIG. 23 is a perspective view thereof.
In measurement of bearings, a +x-direction bias is applied by one bias coil 101 (referred to as an x-direction coil) to the four magneto resistive elements 91, 92, 93, and 94 constituting the MR bridge to measure an intermediate potential difference among the magneto resistive elements, and then, a xe2x88x92x-direction bias is applied by the same bias coil 101 to the magneto resistive elements to measure the intermediate potential difference among the magneto resistive elements. A difference between the intermediate potential differences measured when the +x-direction bias is applied and when the xe2x88x92x-direction bias is applied is proportional to sin xcex8, the angle xcex8 being an angle between the horizontal component of the earth magnetism and the x-axis.
Next, a +y-direction bias is applied by the other bias coil 102 (referred to as a y-direction coil) to the four magneto resistive elements 91, 92, 93, and 94 constituting the MR bridge to measure an intermediate potential difference among the magneto resistive elements, and then, a xe2x88x92y-direction bias is applied by the same bias coil 102 to the magneto resistive elements to measure the intermediate potential difference among the magneto resistive elements. A difference between the intermediate potential differences measured when the +y-direction bias is applied and when the xe2x88x92y-direction bias is applied is proportional to sin(xcfx80/2xe2x88x92xcex8), that is, cos xcex8.
From the y-directional output Vy and the x-directional output Vx, the bearings can be measured as the direction xcex8 of the horizontal component of the earth magnetism as follows:
xcex8=tanxe2x88x921(Vx/Vy).
However, the relationship between the magnetic field applied to the magneto resistive element and the resistance practically involves a hysteresis as shown in FIG. 24, rather than FIG. 20. When the applied magnetic field strength H is increased, it reaches a level of saturation via the upper curve in FIG. 24, and when it is decreased from the level, it traces the lower curve.
Therefore, when measuring bearings, the saturation magnetic field is applied before the application of the biasing magnetic field in consideration of the hysteresis.
For example, as disclosed in Japanese Patent Laid-Open No. 5-157565, when measuring bearings using the azimuth meter composed of the magneto resistive elements and two orthogonal bias coils as described above, the saturation magnetic field Hk is applied in +x direction, and then the intermediate potential difference between the magneto resistive elements is measured while applying the +x-direction biasing magnetic field Hb. Then, the saturation magnetic field xe2x88x92Hk is applied in xe2x88x92x direction by the same bias coil, and then the intermediate potential difference between the magneto resistive elements is measured while applying the xe2x88x92x-direction biasing magnetic field xe2x88x92Hb. The difference between the intermediate potential differences at the time of applications of the +x-direction bias and the xe2x88x92x-direction bias thus obtained is defined as an x-direction output Vx.
Then, the saturation magnetic field is applied in the +y direction by the other bias coil, and then the intermediate potential difference between the magneto resistive elements is measured while applying the +y-direction biasing magnetic field. Then, the saturation magnetic field is applied in the xe2x88x92y direction by the same bias coil, and then the intermediate potential difference between the magneto resistive elements is measured while applying the xe2x88x92y-direction biasing magnetic field. The difference between the intermediate potential differences at the time of applications of the +y-direction bias and the xe2x88x92y-direction bias thus obtained is defined as an y-direction output Vy. Based on the Vx and Vy, bearings are measured in the manner as described above.
The orthogonal four magneto resistive elements assembled into the MR bridge described above may be formed as zigzag magneto resistive elements formed by etching a Ni-based alloy film deposited on a ceramic substrate. Thus, the magneto resistive elements can be quite small and thin. However, since the two bias coil wound around them in x direction and y direction are provided outside the magneto resistive element bridge, the azimuth meter has, at the smallest, a thickness of the order of 3 mm and an area of the order of 10 mmxc3x9710 mm.
In the procedure of measuring bearings explained in the above description, it is required to carry out measuring four times because the bias is applied in +x direction and xe2x88x92x direction by the x-direction coil, the bias is applied in +y direction and xe2x88x92y direction by the y-direction coil, and then calculation is carried out.
Furthermore, in order to eliminate the effect of the hysteresis, before the biasing magnetic field is applied, the saturation magnetic field of the same direction as that of the biasing magnetic field is applied. After the application of the saturation magnetic field, application of the biasing magnetic field of the same direction tends to make the gradient of the curve for the resistance of the magneto resistive element, and the magnetic field be decreased, so that the output to be measured becomes low.
An object of the present invention is to provide an azimuth meter or a bearing sensor of a significantly reduced thickness and area.
Furthermore, another object of the present invention is to provide an azimuth meter or a bearing sensor in which the number of applications of a current to a coil and the number of measurements are less than before.
For example, an azimuth meter according to the invention comprises: a plane coil wound into a rectangular shape; and at least two groups of thin film magneto resistive elements disposed substantially parallel to the plane of the plane coil, in which each of said groups of magneto resistive elements constitutes an MR bridge of an even number of magneto resistive elements electrically connected to each other and detects and outputs two perpendicular components of the earth magnetism, and bearing information is obtained based on the output values, wherein the azimuth meter further comprises: means of passing a current in a predetermined direction through the plane coil to apply thereto a magnetic field that is equal to or higher than a saturation magnetization of said magneto resistive elements, applying a constant biasing magnetic field in the direction opposite to the direction, applying a magnetic field equal to or higher than the saturation magnetization of the magneto resistive elements in the direction opposite to said predetermined direction, and then applying a biasing magnetic field in the direction opposite to the latter direction; and means of passing a magnetic field measuring current through the groups of the thin film magneto resistive elements concurrently with the applications of said biasing magnetic fields.
Preferably, a circuit is arranged so that, when one power supply is used to apply the magnetic field equal to or higher than the saturation magnetization of the magneto resistive elements, a discharge voltage of a capacitor having been previously charged by a shunt current from the power supply is superimposed to the voltage applied to the plane coil.
Preferably, an angle xcex2 formed between a longitudinal direction of each magneto resistive element and a side of the plane coil in the vicinity of the magneto resistive element satisfies a relation of sin xcex2xc3x97cos xcex2xe2x89xa00, and an applied magnetic field characteristic in the vicinity of a region where the electrical resistance variation of the magneto resistive element in response to the applied magnetic field is the minimum is used.
Preferably, one of the groups of thin film magneto resistive elements is constituted by two pairs of magneto resistive elements, the magneto resistive elements in each pair being disposed intersecting opposite sides of said rectangular coil and being electrically connected to each other, the other of the groups of thin film magneto resistive elements is constituted by two pairs of magneto resistive elements, the magneto resistive elements in each pair being disposed intersecting opposite sides, different from said sides, of said rectangular coil and being electrically connected to each other, and the longitudinal directions of two magneto resistive elements disposed on a same side are substantially perpendicular to each other.
Preferably, the angle xcex2 is any of about 45 degrees, about 135 degrees, about 225 degrees and about 315 degrees. Preferably, the variation of the angle at which the longitudinal direction of each magneto resistive element intersects the side of the rectangular coil falls within a range of xc2x15 degrees. If necessary, in the case where the two perpendicular components of the earth magnetism are detected by each of the groups of magneto resistive elements and output therefrom, and the bearing information is obtained based on the output values, a circuit may be additionally provided which outputs a difference between an output obtained when a bias is applied in a positive direction and an output obtained when a bias is applied in a negative direction.
A procedure of passing a current through the plane coil in a predetermined direction to apply thereto a magnetic field that is equal to or higher than a saturation magnetization of said magneto resistive elements, applying a constant biasing magnetic field in the opposite direction, and then measuring the resulting magnetic field to obtain an output value and a procedure of applying a magnetic field equal to or higher than the saturation magnetization of the magneto resistive elements in the direction opposite to said predetermined direction, applying a biasing magnetic field in the opposite direction, and then measuring the resulting magnetic field to obtain an output value may be performed two or more times, and the bearing information may be obtained based on the output values.
In the azimuth meter according to the invention, an even number of magneto resistive elements are electrically connected to each other to constitute an MR bridge. For example, in the xe2x80x9cMR bridgexe2x80x9d in the invention, magneto resistive elements A and B are disposed on opposite sides of a plane coil wound in a rectangular shape and connected in series, a magneto resistive element C perpendicular to the magneto resistive element A is disposed on the same side as the magneto resistive element A, a magneto resistive element D perpendicular to the magneto resistive element B is disposed on the same side as the magneto resistive element B, and the magneto resistive elements C and D are connected in series. And, the bridge is arranged to output a potential difference between an output V1 at the midpoint between the magneto resistive elements A and B and an output V2 at the midpoint between the magneto resistive elements C and D. According to the invention, two generally perpendicular components of the earth magnetism are detected by each of the groups of magneto resistive elements, and the bearing information is obtained based on the outputs thereof. This can reduce the effect of a hysteresis of an applied magnetic field on a resistance, eliminate the noise in the output and increase the absolute value of the output.